Nitrous oxide cycling in the water column and sediments of the oxygen minimum zone, eastern subtropical North Pacific, Southern California, and Northern Mexico (23°N–34°N)

Identifying sources and sinks of N 2 O can illuminate N cycling processes in marine systems, particularly where changes in dissolved O 2 can lead to changes in N cycling pathways (i.e., nitrification versus denitrification). We measured N 2 O andNO 3 −concentration and their stable isotope ratios (δ 15 N and δ 18 O) in the water column and sediments of the oxygen minimum zone in the nearshore eastern subtropical North Pacific (23°N–34°N). Atmospheric efflux of N 2 O ranged from 2.2 to 17.9 μmol m −2 d −1 or about 2–20 times higher than in oxygenated regions of the North Pacific. Surface waters were a source of 15 N‐depleted and 18 O‐enriched N 2 O to the atmosphere, indicating a bacterial, not archaeal, nitrification N 2 O source. Stable isotopes indicated that nitrification in both surface and intermediate waters (∼0–200 m) was the major source of N 2 O in this study area, with denitrification acting as a small N 2 O sink in strongly O 2 ‐depleted waters. Denitrification had a larger impact on observed patterns of N 2 O andNO 3 −concentrations and isotope ratios in the southern oxygen minimum zone. Sediments were generally neutral or a weak sink for N 2 O, with only one site (Soledad basin) showing a positive efflux of +3.5 ± 1.0 μmol N 2 O‐N m −2 d −1 . Sediment fluxes of N 2 O at all sites were several orders of magnitude smaller than fluxes of dinitrogen, nitrate, and ammonium measured in previous studies and did not appear to impact water column N 2 O concentrations. N 2 O was less than 0.1% of the N 2 efflux from sedimentary denitrification.